Group-III group-V compound semiconductors (often referred to as III-V compound semiconductors) such as gallium nitride (GaN) and their related alloys have been under intense research in recent years due to their promising applications in electronic and optoelectronic devices. Particular examples of potential optoelectronic devices employing III-V compound semiconductors include blue light emitting diodes and laser diodes, and ultra-violet (UV) photo-detectors. The large bandgap and high electron saturation velocity of many III-V compound semiconductors also make them excellent candidates for applications in high temperature and high-speed power electronics.
Epitaxially grown GaN films are widely used for in the fabrication of light-emitting diodes. Unfortunately, epitaxial GaN films must be grown on substrates other than GaN because it is extremely difficult to obtain GaN bulk crystals due to the high equilibrium pressure of nitrogen at the temperatures typically used to grow bulk crystals. Owing to the lack of feasible bulk growth methods for GaN substrates, GaN is commonly deposited epitaxially on dissimilar substrates such as silicon, SiC and sapphire (Al2O3). However, the growth of GaN films on dissimilar substrates is difficult because these substrates have lattice constants and thermal expansion coefficients different than that of GaN. If the difficulties in the growth of GaN films on silicon substrates could be overcome, silicon substrates would be attractive for GaN growth given their low cost, large diameter, high crystal and surface quality, controllable electrical conductivity, and high thermal conductivity. The use of silicon substrates would also provide easy integration of GaN based optoelectronic devices with silicon-based electronic devices.
Additionally, due to the lacking of appropriate substrates for growing GaN films thereon, the sizes of the GaN films are limited. The high stresses created by growing a GaN film on a dissimilar substrate may cause the substrate to bow. This may cause several adverse effects. Firstly, a great number of defects (dislocations) will be generated in the supposedly crystalline GaN films. Secondly, the thicknesses of the resulting GaN film will be less uniform, causing wavelength shifts of the light emitted by the optical devices formed on the GaN films. Thirdly, cracks may be generated in large stressed GaN films.